Quartz crystal oscillators are frequently used to stabilize the frequency of electronic circuits. A quartz crystal acts as a stable mechanical resonator, which, by its piezoelectric behavior and high Q determines the frequency generated in an oscillator circuit. Various quartz crystal cuts, vibrating in a variety of modes are used to cover a wide range of frequencies. The Q values of quartz crystal units are much higher than those attainable with other conventional circuit elements.
Those concerned with the design of piezoelectric oscillators have consistently sought to discover new crystal cuts which have a high intrinsic Q. Furthermore, those concerned with the design of systems such as coherent radar, coherent communication systems, and phase-locked loops have consistently sought to reduce the noise inherent in such systems.
It has been determined empirically by J. J. Gagnepain et al., in "Excess Noise In Quartz Crystal Resonators," Proc. 37th Frequency Control Symposium, June 1983, pp. 218-225, that one form of noise, the close-in phase noise, i.e. flicker of frequency, or l/f noise, is related to the intrinsic Q of a quartz crystal resonator by: ##EQU1## where S.sub.yo is the l/f noise power one Hertz from the carrier. Thus, l/f noise depends upon the resonator's unloaded Q according to a l/Q.sup.4 law.
Quartz exhibits a variety of thickness modes of vibration. Three of the thickness modes of vibration are designated the a, b, and c modes. The a mode is a quasi-longitudinal mode. The b mode is a fast quasi-shear mode; the c mode is a slow quasi-shear mode. The intrinsic Q of the b mode is the highest possible in quartz; it can be up to twice that of the c mode, and 10 to 20 percent larger than that of the a mode.
It has been determined in Sinha, "Stress Compensated Orientation for Thickness-shear Quartz Resonators," Proc. 35th Annual Frequency Control Symposium, May 1981, pp. 340-344 that there are stress and temperature compensated quartz cuts which utilize the b mode of vibration. Such cuts would retain the desirable properties of the SC cut vibrating in the c mode while having the advantage of a greater intrinsic Q.
However, if the crystal cuts described by Sinha are fabricated with conventional thickness-field electrodes (i.e. a single electrode on top and bottom of the crystal) both the b and c mode will be excited. Since the b mode and c mode occur at approximately the same frequency, interference and resulting degradation of oscillator performance will occur.
Another method of crystal excitation is termed "lateral field excitation." Lateral field excitation is extensively discussed in U.S. Pat. No. 4,625,138, entitled "Piezoelectric Microwave Resonator Using Lateral Excitation," issued to A. Ballato. Lateral field excitation is also disclosed in a copending patent application entitled "Piezoelectric Resonators Having a Lateral Field Excited SC Cut Quartz Crystal Element, " Ser. No. 738,697, filed by A. Warner and assigned to the same assignee as the present invention.
Those concerned with precision frequency control and quartz oscillator development have consistently sought devices which exhibit high Q and low l/f noise.